Albert Einstein, together with Theodor Kaluza and Oskar Klein, realized that extra dimensions can be used to unify the different fields of physics, as well as unifying the fields with their material sources. In fact, it was Einstein's dream to transpose the "base wood" of the matter term in his field equations to the "marble" of the geometrical term. During his lifetime, this kind of unified theory achieved only partial success. But the modern approach, outlined in this bestseller, is elegant and agrees with all the classical tests. The basic idea is to unify the source and its field using the rich algebra of higher-dimensional Riemannian geometry. In other words, space, time and matter become parts of geometry.